The present invention is generally directed to organic microelectronic devices, and more specifically, in embodiments to the use of a class of polythiophenes as active materials in thin film transistors. The polythiophenes selected can be comprised of repeating thienylene units in which only certain thienylenes possess side chains, and which thienylene units are arranged in a regioregular manner on the polythiophene backbone.
The polythiophenes are in embodiments substantially stable enabling their device fabrication to be accomplished at ambient conditions, and wherein the devices provide higher current on/off ratios, and are operationally more stable as their performance usually does not degrade as rapidly as those of known regioregular polythiophenes such as regioregular poly(3-alkylthiophene-2,5-diyl). More specifically, the polythiophenes of the present invention contain in embodiments repeating segments of 3,4-disubstituted-2,5-thienylene units flanked by unsubstituted 2,5-thienylene units and an optional divalent linkage. The side chains assist in inducing and facilitating molecular self-organization of the polythiophenes during film fabrication, while the unsubstituted thienylene units and the optional divalent linkage, which have some degree of rotational freedom, can disrupt the extended π-conjugation along the polythiophene chain, thus suppressing its propensity towards oxidative doping.
Semiconductive polymers like certain polythiophenes, which are useful as active semiconductor materials in thin film transistors (TFTs), have been reported. A number of these polymers have reasonably good solubility in organic solvents and are thus able to be fabricated as semiconductor channel layers in TFTs by solution processes, such as spin coating, solution casting, dip coating, screen printing, stamp printing, jet printing, and the like. Their ability to be fabricated via common solution processes would render their manufacturing simpler and cost effective as compared to the costly conventional photolithographic processes typical of the silicon-based devices such as hydrogenated amorphous silicon TFTs. Moreover, desired are transistors fabricated with polymer materials, such as polythiophenes, referred to as polymer TFTs, include excellent mechanical durability and structural flexibility, which may be highly desirable for fabricating flexible TFTs on plastic substrates. Flexible TFTs would enable the design of electronic devices which usually require structural flexibility and mechanical durability characteristics. The use of plastic substrates, together with an organic or polymer transistor component, can transform the traditionally rigid silicon TFT into a mechanically more durable and structurally flexible polymer TFT design. The latter is of particular appeal to large-area devices, such as large-area image sensors, electronic paper and other display media as flexible TFTs, could enable a compact and structurally flexible design. Also, the selection of polymer TFTs for integrated circuit logic elements for low-end microelectronics, such as smart cards and radio frequency identification (RFID) tags, and memory/storage devices may also greatly enhance their mechanical durability, thus their useful life span. Nonetheless, many of the semiconductor polythiophenes are not stable when exposed to air as they become oxidatively doped by ambient oxygen resulting in increased conductivity. The result is larger off-current and thus lower current on/off ratio for the devices fabricated from these materials. Accordingly, with many of these materials, rigorous precautions have to be undertaken during materials processing and device fabrication to exclude environmental oxygen to avoid oxidative doping. These precautionary measures add to the cost of manufacturing, therefore, offsetting the appeal of certain polymer TFTs as an economical alternative to amorphous silicon technology, particularly for large-area devices. These and other disadvantages are avoided or minimized in embodiments of the present invention.